The invention relates to a switched power supply having a rectifier for generating a d.c. voltage from an input a.c. voltage and a converter arrangement having a switching transistor and transformer.
Switched power supplies are presently employed to a great degree for power supply systems, since they are lighter, smaller, and cheaper to manufacture in comparison to 50 Hz power supplies.
Universally known switched power supplies comprise a transformer whose primary-side winding is charged with energy by means of a switching transistor and at whose secondary-side winding or windings this energy is taken in the form of one or more constant voltages. The transformer is a component of a flow transformer or of a blocking oscillator. The structure and functioning of these transformer types are extensively described in the book "Schaltnetzteile" by Joachim Wustehube, 1979, Expert-Verlag, Chapter 2.1, incorporated herein by reference.
Switched power supplies usually work in a frequency range from 16 to 500 kHz. Since the occurring current or voltage curves have large overshooting components and, moreover since high currents and voltages must be switched, one must still count on considerable disruptions in the frequency range above 10 kHz (long wave). There are binding laws and regulations for the frequency response of these disruptions relative to the possible propagation in public networks or broadcast into the airwaves.
For the suppression of asymmetrical noise voltages in the network, transformers in switched power supplies have a shielding between the primary and the secondary side. The shielding is composed, for example, of a copper foil which is placed over the primary winding in insulated fashion.
Blocking oscillator power supplies comprise a transformer with an air gap. Particularly given high powers, high eddy current losses arise in the transformer cores of the transformers and in the shielding. It is therefore generally standard (see, for example Wustehube) to employ a so-called conductive fleece for the shielding instead of the copper foil. The manufacture of such a transformer for a blocking oscillator is therefore involved and expensive.
Known flow transformers comprise essentially fixed-clock-controlled, periodically functioning switching transistors as well as a transformer without an air gap whose primary winding is in the collector circuit of the switching transistor and at whose secondary winding a DC voltage can be tapped via a rectifier arrangement. Since a prescribed sampling frequency is used given this circuit principle, the point in time of the cut-in of the switching transistor is predetermined. This circuit principle has the disadvantage that the switching transistor is switched on under power and a free-wheeling diode in the secondary circuit switches into the inhibited condition under power. This leads to high radio interference voltages on the network line. When these converter circuits are utilized in text terminal equipment of communication technology with electrical separation according to protective class 2, i.e. are used without a grounded conductor, then these devices cannot be radio-shielded to the required radio interference level as set by a limiting value in class B.
It is known in switched power supplies for maintaining the output voltage output at the secondary side, to regulate the ratio of on time to off time, or the ratio of on time to the sum of on time and off time, i.e. to regulate the so-called pulse duty factor for the switching transistor.